Reciprocating pumps are commonly used in applications where high volumes of fluid need to be pumped at high pressures. Highest efficiency reciprocating pumps are commonly driven by a crank mechanism. Since any given crank mechanism provides for a constant stroke of the plunger, two methods are typically used for controlling volumetric flow of these pumps:                changing the liner/plunger size, since liners and plungers of smaller diameter fill provide for less flow per stroke of the pump and vice versa,        changing the speed of the pump, since slowing down the pump will result in fewer “strokes per minute” which will result in less flow of fluid and vice versa.        
Both methods have limitations. Changing the liners and plungers is labour-intensive and requires skilled labour, and the pump must be taken out of service during changing, resulting in “down time” of the pump. Controlling the speed may be practical within a limited range if a diesel or gas engine is being used as a prime mover. However, if an electric motor is used as a prime mover, very expensive and inefficient speed controls need to be used, such as VFD or DC controllers.
In order to achieve an appropriate match between pumping requirements and capabilities of the engines used as prime movers, expensive, multispeed transmissions are often used, especially in frac-pump applications where they are often 6, 7 or even 8-speed units. Mud-pumping applications often use 2-speed transmissions. Furthermore, shifting between speeds can result in momentary interruptions in fluid flow.
There is a need for improved apparatus and methods for controlling the volumetric flow of fluid discharged from reciprocating pumps.